1. Field of the Invention
This invention relates to laser beam boresight systems and particularly to the angular displacement of the laser beam from a reference axis.
2. Description of the Prior Art
Electro-optical laser designators are used in laser guided weapons delivery systems to illuminate a selected target in a target scene with a beam of energy from a laser. The reflection of this illumination is used by the weapon system to guide a weapon to the selected target. Electro-optical laser designators may employ an image sensor to view and track a desired target in the field of view. A target may be selected by an operator or by a screening circuit. The target position in the scene or field of view is centered or referenced to the boresight axis of the image sensor. As is well known, precise alignment of the laser beam axis with the boresight axis of the image sensor requires aligning the laser beam axis to within tenths of a milliradian, otherwise errors in the desired and actual target designation will occur causing error in weapons delivery accuracy.
The laser beam may be invisible to both the operator and the image sensor preventing direct monitoring of the alignment of the laser beam axis.
Even in systems with precise alignment initially, errors in alignment of the beam of energy repetitively emitted by the laser are likely to occur because of unintended angular displacements of the successive laser beam caused by stresses in the laser and its supporting structure arising from mechanical and thermal effects. One approach employed to maintain precise alignment and to reduce alignment errors is to construct the structure rigid enough so that deflections under severe dynamic loads would be greatly limited. The weight of the structure needed to provide the required rigidity is significant. For some applications such as in man-portable and in small remotely piloted vehicle (RPV) systems, the weight of the system is a critical parameter.
One approach to the alignment of a laser beam is the technique disclosed in U.S. Pat. No. 4,155,096 issued May 1979 to Thomas, et al. Thomas describes a system wherein the laser beam itself provides the boresight reference axis. When the boresighting procedure is to be employed, the laser beam is temporarily shifted from the output port and target scene to a different direction and a small portion of the beam is permitted to be transmitted through an optical system and focused as a spot image on the faceplate of an imaging sensor. A target tracker is then locked-on to the laser spot and, when the system has stored the position information regarding the image, the laser beam is redirected to the output port. While the laser beam is directed to the output port, no portion of the beam is transmitted to the sensor. The tracker will utilize that laser image as the boresight position of the laser beam on the presumption that no changes will occur to the beam's alignment during the period of time the laser is directed away from the sensor. However, mechanical and thermal stresses will ordinarily continually occur to the laser and its supporting structure which could affect the alignment of the laser beam. The inability to continually monitor and update the alignment of the laser beam during the tracking mode appears to be a limitation of that system. An additional limitation is that other operating modes must be inhibited until such a time as the boresighting mode has been completed; also the frequency of the laser beam must be within the frequency response spectrum of the imaging sensor.
Another approach to the alignment of a laser beam with respect to a target is described in U.S. Pat. No. 4,015,906 issued on Apr. 5, 1977 to Uzi Sharon. In U.S. Pat. No. 4,015,906, a portion of the laser beam is reflected and focused onto a surface of a plate which glows when impinged by a laser beam. A microscope which is aligned on an axis parallel to but spaced laterally from an axis of the laser beam receives by means of a beam splitter radiant energy from the glowing spot on the plate indicative of the position of the laser beam in the field of view of the microscope. The system enables laser boresighting on a target in a scene or field of view prior to exposing the target to the laser beam by opening a shutter.
With regard to measuring alignment, a beam alignment sensor for measuring the angular alignment and linear displacement of a collimated beam of light relative to a fixed reference surface is described in U.S. Pat. No. 3,942,894, issued on Mar. 9, 1976 to Dennis A. Maier. In U.S. Pat. No. 3,942,894, an annular mirror provides a fixed reference member with which the main beam is aligned utilizing light reflected from the beam which is focused on a mirrored prism to determine angular misalignment. Another sensor determines the lateral displacement of the beam portion directed to the mirrored prism.
It is therefore desirable to provide a system for automatic and continuous updating of the alignment of the laser beam axis each time the laser emits a beam of energy.
It is further desirable to provide a system in which the frequency response range of the imaging sensor may be independent of the laser frequency so that the same laser frequency can be employed whether, for example, a TV sensor is to be utilized for daylight operation or a Thermal Imaging sensor is to be utilized for operation under low light level conditions.
It is further desirable to provide a system wherein the rigid supporting structure formerly required to overcome deformation effects on the alignment position of the laser beam is not required.